Child-proofing is the first thing I'd think of. Not wanting to walk 2-3 times I still remember carrying loads of stuff just by their cables and I'll spare you my memories as Janitor on a school for kids aged 4-12
The way you at-/detach motors/sensors etc seems indeed important.
Having played with Fischertechnik myself as a kid I noticed it still exists, but... it's also expensive.
Second hand would of course be nice.
One thing I regret as a child was little knowledge about working with wood/metal.
I wouldn't advice a small child to operate heavy duty machinery, but I probably would have built a lot more with just a load of old aluminum + tap&die-set.
Re-using would be a little more difficult, but at the same time a nice mental challenge.